Public address stabilizing system



Feb. 6, 1940. J. ANDERSON PUBLIC ADDRESS STABILIZING SYSTEM Filed Nov. 28, 1936 rm 1 O M 4 N m a 0 m m I 3 G H a an .n 1.... N h 6 4 e I It N m i G l S. I m 1 6 m W F a S E \L. 4 E

9 "a n 1 a z w I I l I ll 2 m 5 MW lunch I! m E T L N P w M H 7- m v. F m w s M H 1 w m W F Km 3 z m |1+|ll :u G 1 T I \(QQN 1 E .m P IIIII 1 PM ZooN f 1 L fin 1 M 2 1 n g E E m 7 1 a Z m Patented Feb. 6, 940

- UNlTEDfST TEs ruBLIo ADDRESS'STABILIZING srsrEM A Leslie J. Anderson, Westmont, N. "J; assignor to Radio Corporation of America, a corporation of Delaware Application November 2s,1QesQseri Nai1a1s1 6 Claims. (01.- 179- 171):

jsound reproducing and pick-up devices, commonly designated as singing results from sound whichfl) is produced byth'e loud speaker,

(2) .is picked up by the microphone, and Y (3) produces a corresponding electrical current which,

is amplified and fed to the loud speaker. If the microphone and loudspeaker are in the same room the amount of sound amplification which can be employed. ispractically alwayslimitedby the feed-back or singing point. Such; singing almost always occursat; the frequency at which the-overall amplification is at a maximum. This phone todistinguish "between feed-back sounds involves the complete circuit-"starting with the microphone and ending with the. sound received by the microphone from the loudspeaker, thus including such sound reinforcement orsound losses as are imparted by the entire acoustical environment. The microphone may be especially sensitive at one frequency, or theloud speaker may be especially efiicient at a particular freq quency," or the amplifier'may have a peak at a certain frequency. All these factors have a part in determining the frequency at which feed-back or singing becomes evident.

It has been found that the usefulness of a sound reinforcing system'is greatly enhanced by minimizing all of thepeaks in the system so that;

the overall characteristicis as nearlynniform with respect to frequency as possible. Further advantage has been taken of the directive proper- 1 tiesof bothmicrophonesand loud speakers in order that the. microphone may receive a mini? l mum ofsound from the loud speaker as compared with the sound which the loud speaker radiates toward the hearers, and the sound which the microphone receives from the original source. In spite of what it has, been possible to accomplish bythe'aforementioned measures, the usefulness of public address systems is still. seriously limitedby feed-back or fsinging.

of a public address system with a slow acting automatic; volume control of the fcompressor? type, for reducing, the amplification when there is any prolonged sound. This would serve to stop feed-back if the sustained sounds were due to feed-back, but no satisfactory arrangement has been proposed that will enable the microand sounds which it is desired to amplify.

A possible solution of the problem is the pro- .LFigure 3 is a similar It has been proposedto provide the amplifier vision of means for causing the amplification to be momentarily increased in fresponse to voice sounds. A quickacting expander circuit having a limited. range 'of volume control may be employed for thispurpose. vThe theoryfof this TOFF C approach to the problem is} that the high lamplification condition does notylast long enough to permit the building uplof feed-backoscillations. l y

In accordance with the present invention, feedback between the sound reproducer and pick-up devices is minimized by the provision of a signal amplifier which is biased to'low gain or substantially to cut off in the absence of signal impulses,

and has its gain increased in. response to the presence of such impulses.) Stated more. specifically asigna-l channel biased substantially to v cut-ofi and fed through a high pass filter is arranged to have its gain controlled by a control circuit which is fed through a low pass filter'and overloads; at a predetermined signal level.

.The. invention will be better understood from the following description when considered in connectionwith the accompanying drawing; and its scope is indicatedby the accompanyingclaims.

. Referring to the drawing? Figure l is a wiring diagram of a public ad dress system embodying the invention,

Figure Z'is an explanatory diagram showing thegainof ;the main signal channel at different frequencies, a

diagram relating'to the operation of the control channel and Figure 4 showsthe relation between the input and output voltages of the control channel.

' The system of Fig.1 includes a sound pick-up device or microphone JD, from which signalsare supplied to a loud speaker I! through a signal channelg'which includes a high pass filter [2; I3,

14, an amplifier stage l5 which maybe of the- RCA Radiotron 58 type, and a-main amplifier l6. .Theamplifier stage I 5 is substantially biased to cut-off by means of a potential source shown asa' battery I! which is connected in series with aresistorl8. M

. Signalimpulses are also supplied from the microphone l0 to acontrol amplifier l9 through a low pass filter 2E], 2!,22. "The outputof the amplifier i9 is rectified by a rectifier 23,and the rectified or detected current is supplied to the resistor 88, an adjustable potential sourceshown as a battery 26, being included in the; output circuit for the purpose-of bucking out small variations'in the rectifier output; The potential produced inthe resistor i8 is of such polarity as to make the control grid of the amplifier l5 more positive and permit the transmission of signals through the main channel. In order to properly time the building up of this bias potential, resistor 24 and capacitor 25 are provided As indicated more clearly by Fig. 2, the gain of the main channel maybe made to increase at a predetermined frequency, such as 200 cycles,

to a predetermined level, and to decrease at a higher frequency, such as 5,000'cycles, for example, this level being determined by overloading of the control channel, and the decrease bequencies.

ing effected by suitable design of the coupling transformers or other parts of the main'chan nel. While a range of 200 to 5,000 cycles has been indicatedfor purposes: of illustration, it will of course be understood that any frequency range may be utilized.

The average male voice contains frequencies preponderantly below 200 cycles and in all speech there is present a'considerable quantity of fre quencies below 200 cycles. In addition to this, all speech contains a Very large percentage of waves which are not true sine waves. Many of these waves are substantially square-fronted waves and therefore produce response at all fre- A velocity microphone is preferably used with speech close thereto.

. istics of this microphone are such that speaking close to the microphone greatly accentuates the response to low frequencies and particularly those below 200 cycles. As aconsequence, when any speech is impressed on the microphone [0 a sufficient quantity thereof produces response below 200 cycles to actuate the control amplifier and rectifier and thereby supply suflicient biasing voltage to the tube IE to render it properly conducting.

If desired, of course, the main channel may carry considerably higher frequencies than 200 cycles as, for example, 300 or 400 cycles, as the presence of low frequencies" is not essential to understanding the human voice. In this. case, the low pass filter would be arranged to carry a correspondingly greater range of frequencies.

As indicated by Fig. 3, the gain of the control channel may be caused to decrease as the frequency increases, a minimum gain being reached at some selected frequency, such as 200 cycles. Under these conditions, the relation between the microphone output voltage and the output voltage of the control circuit is as indicatedby the curve of Fig. 4. Y

Under these conditions, no feed-back between the microphone and speaker is possible; Thus, when speech is picked up, in the microphone, the control channel is actuated in such away as to. apply a bucking bias to the stage 15 of the main amplifier. This stage then functionsv as an arm plifier, and as a result the amplified speech impulses are heard at the speaker. The loud speaker is prevented from actuating the control channel by the filter 20, 2|, 22. The output from the rectifier 23 is maintained substantially constant during the transmission of signals, due to the fact that it overloads when a predetermined signal level has been reached. It has been found that the use of a velocity microphone is advantageous in this type of system, for the reason that in close talking the large velocity effect The character voltage may be used advantageously in the control channel. 7

I claim, as my invention:

1.' In a system including an amplifier connected between acoustically coupled sound pick-up and reproducing devices, the combination of means for restricting the input of said amplifier to impulses of relatively high frequency, means for biasing said amplifier to a low signal level, and means responsive to relatively low frequency impulses forapplying a counter-bias to said amplifier. I

2. In a system including an amplifier connected between acoustically coupled sound. pick-up and reproducing devices, the combination of means for restricting theinput of said amplifier to the relatively high" frequency components of the output of said pick-up device, a control channel for controlling the gain of said amplifier, and means for 'restric'ting the input of said control channel to the relatively low frequency components. y

3. In a system including an amplifier connect ed between acoustically coupled sound pick-up and reproducing devices, the combination of means for restricting the input of said amplifier to the relativelyhigh frequency components of the output of 'said pick-up device, a control channel for controlling the gain of said amplifier,

means for restricting the input of said control channel to the relatively low frequency components of the output of said pick-up devices, and means for timing the action of said control channel.

4. In a system including an amplifier connect- 5. In a system including an amplifier con nected between acoustically coupled sound pickup and reproducing devices, the combination of means for restricting the input of said amplifier to the relatively high frequencycomponents of the output of said pick-up device, a control channel for controlling the gain of said amplifier, and

means for restricting the input of said control channel to the relatively low frequency com ponents of the output of said pick-updevices,

said control channel being arranged to overload at a predetermined level of said relatively low' frequency components. v

6;In a system including an amplifier connected between acoustically coupled sound pick-up" and reproducing devices, the combination of a signal channel including a high pass filter, means for biasing said channel substantially to cut-off, a control channel including a low pass filter and arranged to apply to said signal channel a counter-bias potential whereby signal impulses are transmitted to said channel only in response to the presence of speech current in the output of said pick-up device.

LESLIE J. ANDERSON. 

